In many industrial environments, it is necessary to detect the level of product in a holding tank or bin. Level sensors are typically attached to the holding tank or bin, and electrically connected to remote gauges at a control room or other central location, where technicians or control systems may monitor the status of the bins to provide the appropriate process control.
Various technologies have been developed for level sensing. These include various contact sensing technologies using floats or drop weights, as well as various non-contact technologies such as reflecting electromagnetic radiation or ultrasonic vibrations from the surface of the product in the bin to determine the height of the product.
In some applications, it is particularly important to move the sensor away from the product. For example, in a foundry where the level of a hot melt of steel or ore is to be level sensed, it is particularly important to keep the level sensor a safe distance from the hot melt. In these applications, nuclear level sensing gauges are used.
In a nuclear level sensing gauge, a source of nuclear radiation is positioned on one side of the bin to be level sensed. A nuclear radiation detector is placed on the opposite side of the bin. The radiation exiting the source is in the shape of a wide generally vertically dispersed beam, directed toward the interior of the bin. The product in the bin substantially absorbs the radiation that impinges upon it. If, however, the bin is not full of product, some part of the beam of radiation from the source passes through the bin and exits from the bin on the side opposite to the radiation source, and irradiates the radiation detector. Because the product in the bin substantially absorbs the radiation that impinges upon it, thus reducing the amount of the radiation beam passing through the bin, the amount of radiation stimulating the radiation detector, is inversely proportional to the amount of product to the bin. Thus, the amount of radiation detected by the radiation detector, is compared to minimum and maximum values to produce a measurement of the amount of product in the bin.
In a typical nuclear level sensing gauge, the nuclear detector is based on a scintillating crystal. A scintillating crystal produces light when exposed to nuclear radiation. The amount of light produced is related to the amount of radiation impinging on the crystal. To detect radiation passing through the bin, an elongated scintillating crystal is placed on the side of the bin opposite to the radiation source, with the long dimension of the crystal generally vertically oriented. A light detector coupled to an end of the crystal, detects light emanating from the scintillating crystal, and produces from this a signal indicative of the amount of radiation impinging on the crystal, and thus the level of product in the bin. This type of sensor is discussed in U.S. Pat. Nos. 3,884,288, 4,481,595, 4,651,800, 4,735,253, 4,739,819 and 5,564,487. Other nuclear radiation detection technologies have also been used in nuclear level sensing gauges, e.g., a Geiger tube is shown in U.S. Pat. No. 3,473,021.
Unfortunately, there are several disadvantages with conventional nuclear level sensing gauges, particularly those gauges using scintillating crystals as a radiation detector. First, an elongated scintillating crystal is bulky, heavy and difficult to ship, as well as expensive to custom manufacture in different lengths. Furthermore, in applications where the scintillating detector must be mounted to a curved bin (e.g., where the bin is a transversely mounted rotating basket), the scintillating crystals must be cut into segments, or custom manufactured with the appropriate curvatures, increasing the expense. Also, scintillating light passing through a relatively long scintillating crystal can be substantially attenuated, reducing the effective signal-to-noise performance of the level sensing gauge.
It will also be appreciated that, in many applications, the scintillating crystal in a nuclear level sensing gauge must be temperature controlled, because the scintillation effect is temperature-dependent. Typically, a cooling system must be associated with the scintillating crystal, to carry away heat generated, for example, by the product in the bin or by another source of heat such as direct sunlight. Scintillating crystals have a relatively large heat capacity, and so are relatively difficult to cool.